Monday, June 18, 2012

CBSE Class 8 - Science - Synthetic Fibres and Plastics

Synthetic Fibres and Plastics

NCERT Chapter Solutions and Q & A

SPI Coding System for Plastics
credits:openclipart.org


Q1: Explain why some fibres are called synthetic?

Answer: Some fibres are called synthetic because they are not made from natural fibres. Instead, they are made by chemical processing of petrochemicals. The synthetic fibres are made up of very large units called polymers. Some of the synthetic fibres are: rayon, nylon and acrylon.







Q2: Mark ( ✓) the correct answer:
Rayon is different from synthetic fibres because
(a) it has a silk like appearance
(b) it is obtained from wood pulp ✓
(c) its fibres can also be woven like those of natural fibres

Answer: (b) it is obtained from wood pulp


Q3: Fill in the blanks with appropriate words:

Answer:

(a) Synthetic fibres are also called man-made or artificial fibres.

(b) Synthetic fibres are synthesised from raw material called petrochemicals.

(c) Like synthetic fibres, plastic is also a polymer.


Q4: Give examples which indicate that nylon fibres are very strong.

Answer: Nylon is a very strong fibre that is why nylon is used for making parachutes, rock climbing ropes, fishing nets seat belts etc. A nylon thread is actually stronger than a steel wire.

Q5: Explain why plastic containers are favoured for storing food.

Answer: Plastic containers are favoured for storing food for the following reasons:
  1. Plastic containers are light weight so they are easy to handle.
  2. The price of plastic containers is very less as compared to other containers.
  3. They have good strength.
  4. Plastic containers are durable.

Q6.Explain the difference between the thermoplastic and thermosetting plastics.

Answer:

Thermoplastics Thermosetting plastics
1 Plasticcs which can be deformed and bend easily on heating are known as thermoplastics. Plastics which when moulded once cannot be bent and deformed on heating are known thermosetting plastics
2 They are used for making toys, containers, etc They are used for making switches, handles of utelsils, floor tiles etc.
3 Example: PVC, Polythene Bakelite and Melamine


Q7.Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Answer:  
(a) Saucepan handles: Saucepan handles are made from thermosetting plastics because these are bad conductors of heat and electricity. They do not bend or deform on heating.

(b) Electric plugs/switches/plug boards: Electric plugs/switches/plug boards etc. are made up of thermosetting plastics because thermosetting plastics are bad conductors of heat and electricity. It does not allow the current to pass through it.



Q8: Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’:
Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Answer:

Can be recycled Cannot be recycled
1 Plastic toys
carry bags
ball point pens
plastic chairs
plastic covering on electrical wires,
plastic bowls
cooker handles
electrical switches
telephone instruments

Q9: Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Answer:  Rana is advised to buy a cotton shirt for summers because cotton absorbs the sweat and the pores in cotton allows easy evaporation and keeps our body cool, whereas synthetic shirts don’t absorb sweat and they are very uncomfortable to wear in summers.

Q10: Give examples to show that plastics are noncorrosive in nature.

Answer:
  1. We can store various kinds of chemicals and other materials in plastic containers because they are non-reactive and does not react with water and air.
  2. Teflon, a kind of plastic is used as a coating to manufacture non-stick cookwares.
  3. Melamine is used for making floor tiles and kitchenware.
  4. Bakelite is a poor conductor of heat and electricity so it is used for making electric switches and handles of the utensils.
Q11: Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Answer:

The bristles of a tooth brush should be made of soft material so that it does not harm the gums and the handle should be made of hard material so that it can give a firm grip. Therefore, the bristles are made up of nylong fibres which are strong, elastic, lustrous, light weight and easy to handle and the handle is made of hard plastic as it is strong, durable, poor conductor of heat and electricity.


Q12: "Avoid plastics as far as possible". Comment on this advice.

Answer:

Although plastics have become important in our daily life, we should avoid them as far as possible. 
  1. Plastics are non-biodegradable and take a lot of time to decompose. 
  2. Plastics are not eco-friendly. By burning plastics, poisonous fumes are released that cause air pollution. 
  3. Sometimes cows and other animals may eat plastics present in the garbage which may choke their respiratory system and even cause death. 
  4. The plastic left in the soil results in soil pollution. 
  5. Plastic bags drained into the water bodies cause threat to the aquatic life also. 
  6. Polybags choke the drains and sewage pipes thus causing lot of trouble. 

So we should use plastic judiciously and give preference to other alternatives like jute bags etc.

Q13: Match the terms of column I correctly with the phrases given in column II.


Column I Column II
i Polyester (a) Prepared by using wood pulp
ii Teflon (b) Used for making parachutes and stockings
iii Rayon (c) Used to make non-stick cookwares
iv Nylon (d) Fabrics do not wrinkle easily

Answer: i-d, ii-c, iii-a, iv-b

Q14: Explain, ‘Manufacturing synthetic fibres is actually helping conservation of forests’.

Answer: 'Manufacturing synthetic fibres is actually helping conservation of forests’ because synthetic fibres are man-made fibres. These fibres are made from petrochemicals. For manufacturing synthetic fibres cutting of trees or killing of animals is not required and hence we conserve our natural environment.

Q15: Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer: Take a small piece of thermoplastic material and connect it as shown in the figure When we close the switch, no current passes and the bulb does not glow.
Q16: Terylene is used in making ...
(a) Dressing material
(b) Food packaging
(c) Non-sticky utensils
(d) All of the above.

Answer: (a) Dressing material

Q17: What are polymers? 

Answer:  Many small chemical units (called macro molecules) combine to form a large unit. Polymer is made up of such repeating units.


Q18: What is the polymer unit of cotton?

Answer: Cellulose.

Q19: What type of polymer units arrangement are found in plastics?

Answer: In general two types:  
  • Linear arrangement
  • Cross-linker arrangement
Q20: Give examples of (a) natural fibres (b) semi-synthetic fibres and (c) synthetic fibres

Answer:

(a) Natural fibres: Cotton, silk, jute
(b) Semi-synthetic fibres: Rayon
(c) Synthetic fibres: PVC, Nylon, Terylene, Teflon

Q21: What is full form of PVC and PET

Answer:  PVC: Polyvinyl Chloride
                PET: Polyethylene tetrapthalate

Q22:  Name the R's in 4R principle.

Answer: Reduce, Reuse, Recycle and Recover.

13 comments:

  1. The answer of que No. 2 is wrong. The correct answer is (b) it is obtained from wood pulp

    ReplyDelete
  2. Hi , Most of the hotels are serving hot food on silver coated paper plates.Can we eat hot food on silver coated paper plates.can u please send your answer to illuri.krishna@gmail.com

    ReplyDelete
  3. This comment has been removed by the author.

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